FIG. 17 shows a bump forming method of the prior art (for example, see Matsushita Technical Journal, Vol. 47, No. 3, Jun. 2001, issued by Panasonic Corporation). The bump forming method uses a wire bonding technique. As shown in FIG. 17A, an Au ball 202 is formed by spark discharge on the end of an Au wire 201 passed through a capillary 203. As shown in FIG. 17B, the Au ball 202 is brought into contact with an electrode 102 of a semiconductor element 101 heated by a heating stage (not shown) and is joined to the electrode 102 by applying a pressure and an ultrasonic wave. As shown in FIG. 17C, the Au wire 201 is pulled off by lifting the capillary 203 while drawing the Au wire 201 by a fixed amount, so that a stud bump 208 is formed.
FIGS. 18A and 18B show the overall configuration of a flip-chip mounting structure and a sectional view of a connected portion of the flip-chip mounting structure according to Japanese Patent No. 3150347. The stud bump 208 formed in the foregoing manner on the electrode 102 of the semiconductor element 101 is connected to an electrode 302 of a printed circuit board 301.
FIGS. 19A to 19F and FIGS. 20A to 20C show a bump forming method described in Japanese Patent Laid-Open No. 8-264540. The bump forming method also uses a wire bonding technique. As shown in FIG. 19A, a metal ball 205 is formed on the end of a wire 204 passed through a capillary 203. As shown in FIG. 19B, the metal ball 205 is joined to an electrode 102 of a semiconductor element 101 by applying an ultrasonic wave while pressing the metal ball 205 to the electrode 102 with the end of the capillary 203. As shown in FIGS. 19C and 19D, the end of the capillary 203 is laterally moved and then an ultrasonic wave is applied while a pressure is applied to the joint of the metal ball 205, so that the wire 204 is cut off as shown in FIG. 19(E). Thus a first bump 206 is formed.
After that, another metal ball 205 is similarly formed and is joined onto the first bump 206 as shown in FIG. 19F. As shown in FIGS. 20A and 20B, the end of the capillary 203 is laterally moved and then an ultrasonic wave is applied while a pressure is applied to the joint of the metal ball 205, so that the wire 204 is cut off. Thus a second bump 207 is formed and a two-layer bump 218 is completed.
In the two-layer bump 218, the first bump 206 and the second bump 207 are substantially identical in shape or the second bump 207 is smaller than the first bump 206. FIG. 21 shows a sectional view of a connected portion of a flip-chip mounting structure using the two-layer bump 218.
The stud bump 208 and the two-layer bump 218 are both formed on the electrode 102 of the semiconductor element 101. Thus the diameters of the bumps have decreased as the electrodes 102 have been reduced in size with higher densities. As the diameters of the bumps decrease, a contact area between the circuit board electrode 302 and one of the stud bump 208 and the two-layer bump 218 decreases in flip-chip mounting, reducing the reliability of connection.